Munitions come in several forms. There are munitions that are launched from airborne platforms, submerged platforms, surface ships, artillery, and tanks to name a few. Regardless of the platform from which a munition is launched, the munition must be guided to the desired target. In many instances, the guidance is largely determined by the conditions imposed upon a munition at the point of launching. For instance, the guidance of a bomb dropped from a bomber during WWII was substantially determined by the speed and altitude of the bomber at the point in time when the bomb was released. The released bomb had little, if any, ability to guide itself to a desired target after being released. Hence, such bombs are typically referred to as “dumb bombs.” More recently, many munitions include guidance systems that, if needed, direct the munition to the desired target by providing appropriate signals to control surfaces associated with the munition. Examples of such munitions are cruise missiles and Joint Direct Attack Munition (JDAM) smart bombs. Many of these guidance systems employ inertial navigation and Global Positioning System (GPS) receivers in guiding the munition to the desired target.
In many instances, the effectiveness of the munition depends not only on being guided to the target but also on detonating at a desired height above the target, which is frequently referred to as height-of-burst (HOB). Generally, there are two types of HOB fuze systems associated with a munition. In the first type of HOB fuze system, the period of time required for the munition to move between a reference point in the anticipated flight path (e.g., the launch point) and the desired HOB point is calculated. Upon expiration of the calculated period of time, the fuze causes a detonation signal to be issued. The calculation is typically quite complicated and may take into account factors such as the launch location, target location, ballistic trajectory, speed of the munition at launch, meteorological conditions etc.
The second type of HOB fuze system that is associated with munitions employs a transmitter and a receiver. The transmitter produces a signal that is directed to and reflected from a surface associated with the target. The receiver receives the reflected signal. Based on the time elapsed between the transmission of the signal and the reception of the reflected signal, the system can calculate the height of the munition above the surface and issue a detonation signal if the munition is at or near the desired HOB. Due to the presence of a transmitter, such systems are typically referred as “active” HOB fuze systems.